In many applications it is important that the bondline thickness of the adhesive between substrates is of a known uniform thickness. However, measuring the bondline thickness after the adhesive has cured is very difficult without destroying the adhesive bond.
In certain applications it is essential that not only should the thickness of the adhesive bond be measurable, but also that the degree of adhesive cover between the substrates be assessed. These factors are of critical importance in applications where the adhesive forms the bondline between a data carrying layer, such as a polycarbonate coated with a metallised reflective layer, and say an optically clear protective layer. Failure of the adhesive to spread over, at least, the data carrying portion of the substrate surface and/or an uneven thickness of adhesive to bond between the substrates can have detrimental effects to the retrieval of data from the chosen substrate.
It has now been found that the adhesive compositions according to the present invention, which comprise at least one light emitting material, can be used to produce a bondline which can be analysed for thickness and/or surface coverage via non-invasive methods.
Thus according to a further embodiment of the present invention there is provided a non-invasive method for the measurement of the adhesive bondline thickness between substrates, at least one of which is optically clear.